Such disc spreaders are in widespread use, in particular, in agriculture for distributing granular material, such as mineral or organic fertilizer and the like, but also in the form of winter service spreaders for distributing gritting salt and/or grit. Their advantages are predominantly their ease of operation and high level of efficiency alongside comparatively low investment costs. In particular in the case of agricultural disc spreaders, for the most part two-disc fertilizer spreaders have become established, these spreaders being equipped with a pair of distributor discs with metering elements assigned thereto.
In order to ensure the granular material is distributed as desired on the ground, in modern disc spreaders of this type it is possible to perform open-loop or closed-loop control of, on the one hand, the metering of the mass flow of the granular material onto the distributor disc which is assigned to a respective metering device and, on the other hand, the distribution of the granular material by the distributor disc or discs. For the latter it is customary to provide devices for changing the spreading ring sector in order to provide the disc spreader for, for example, different working widths, granular materials and/or types of distribution.
Moreover, however, for the quality of the spreading result in the case of disc spreaders of the generic type which are equipped with rotating distributor discs a determining factor is also to distribute the granular material as uniformly as possible on the ground, such as, for example, the area of a field on which material is to be spread. After the granular material which has been placed on the rotating distributor disc by its metering device has been accelerated by said distributor disc towards its circumference, the granular material is ejected in a fan-like fashion over a specific ejection range, after which the granular material then drops to the ground at a distance from the distributor disc in an essentially annular-segment-shaped range, referred to as the spreading ring sector. In this context, the mass distribution of the granular material is transverse with respect to the direction of travel of the centrifugal spreader, that is to say not uniform over its spreading width but instead the mass distribution generally decreases from a maximum in the central area towards both sides, specifically the essentially radial boundaries with respect to the distributor disc. By arranging one next to the other two distributor discs which are driven in opposite directions, as is customary in the case of two-disc spreaders of the generic type, the transverse distribution of the granular material can be homogenized in the area between the distributor discs and at a short lateral distance from the same, but nevertheless it decreases towards both sides of the entire spreading pattern which is obtained in this way. In the case of fertilizer spreaders this is compensated by what is referred to as the overlap method in which the spreading patterns are overlapped at the edge area. This consequently results in what is referred to as a working width which corresponds to the distance between the lanes used in the overlap method and which is basically smaller than the (total) spreading width which can be, for example, approximately twice as large as the working width. In contrast, in other applications such as, in particular, in the case of winter service spreaders it is attempted to achieve a spreading pattern which has edges which are as steep as possible since an overlap method is not possible here.
In the case of a disc spreader the transverse distribution of the granular material is firstly determined decisively by the spreading ring sector which is generated by its—or by each—distributor disc, specifically by the geometry and spatial position thereof. These are properties which are in turn decisively influenced by the physical of the granular material, specifically on the one hand by the sliding behavior of the particles of the granular material on the disc, on the other hand by the flight behavior thereof after they leave the disc. In this way, particles which slide well, and are therefore relatively smooth and round, leave the disc earlier than relatively rough and sharp-edged particles. In a polar coordinate system with the axis of the distributor disc as a center point, the spreading ring sector therefore migrates in the direction of rotation the rougher and more sharp-edged the particles. On the other hand, the average flight distance of small, lightweight, sharp-edged or rough particles is shorter than that of large, smooth or heavy particles, with the result that in the first case the spreading ring sector is closer to the distributor disc and also has a shorter radial extent than in the second case.
Apart from the granular-material-specific influencing factors, structural parameters also play a role. For example, the centering angle of the spreading ring sector is influenced by the mass flow of granular material which impacts on the distributor disc, i.e. by the respective setting of the associated metering device, in the sense that it becomes larger the larger the mass flow of granular material or the greater the extent to which the metering device is opened. Furthermore, the point at which the granular material is placed on the distributor disc influences the position of the spreading ring sector to the extent that it migrates further counter to the direction of rotation the further the placing point is away from the axis. Furthermore, the spreading ring sector can be “rotated” about the rotational axis of the distributor disc if the placing point of the granular material is moved in the circumferential direction of the distributor disc. Finally, the rotational speed of the distributor disc or discs also determines the extent and position of the spreading ring sector, wherein the faster the distributor disc rotates, the larger the spreading ring sector and the further away it is from the distributor disc.
Since the granular-material-specific properties, that is to say, in particular, the sliding properties and flight properties of the particles of granular material (fraction) which are used, can be controlled the least and are most likely to be subject to fluctuations, in the fertilizing technology use is made of what are referred to as spreading tables, in which, for a specific granular material, corresponding settings for achieving specific working widths and spreading quantities at the disc spreader, that is to say for example the rotational speed of the distributor discs, the position of the point at which the granular material is placed on the distributor discs, the size of the metering opening of the metering device etc. can be read off in order to distribute a specific quantity of the granular material over a specific working width. These spreading tables are recorded in spreading trials and always apply only to the specifically investigated granular material and the spreader used in the context. Since, on the one hand, the granular materials are not standardized and consequently have changing properties (for example the physical properties of usually hygroscopic mineral fertilizer particles are therefore capable of changing hugely in the presence of moisture), and on the other hand the disc spreaders also differ from one another in structural terms it is frequently necessary, in certain circumstances before the spreading work, firstly to perform what is referred to as a turning off test in order to correct the setting of the metering opening of the metering device in accordance with the result of the turning off test and subsequently carry out a spreading test under operating conditions in order to optimize the spreading pattern or the actual distribution of the granular material on the ground in approximately annular-segment-shaped boundaries by means of setting measures at the disc spreader. However, even this often does not bring about a uniform transverse distribution of the granular material because the distribution is influenced by further factors during the spreading operation (subsequently), for example the inclination situation of the spreader on uneven ground, by influences of the wind, the exposure of the granular material to air humidity or even rain, by possible changes, mentioned above, in the physical properties of the granular material etc.
In the prior art it has already been recognized that eliminating these deficiencies is only possible by recording current spreading states during the spreading work in order to be able to intervene in a corrective fashion in the setting or control of the disc spreader when deviations are detected. It is therefore known, for example, to arrange two or more impact sensors on the circumference of the distributor disc in order to detect the general position of the ejection range, and to record a measurement variable which is representative of the number of particles per time unit (EP 0 682 857 A1). In a specific case, the impact sensors are embodied in a tubular shape and arranged on a carrier which can be adjusted manually in a transverse direction, wherein the sound emitted by the tube is measured with a sonotrode. In this context, it is assumed that a type of Gaussian normal distribution with a maximum quantity in the center is present within the spreading ring sector with the result that if the sensor is located in the region of the greatest particle density, which corresponds to the greatest number of impacts, the center of the spreading ring sector has been detected. By changing the setting of the spreader (moving the placing point, changing the rotational speed etc.), the maximum is moved into the desired position and it is then assumed that the spreading ring sector is also located in the desired position. However, in this way it is not possible to detect the transverse distribution of the granular material over the entire ejection range and this method ignores, in particular, the fact that the maximum quantity does not necessarily have to be located in the centre of the spreading sector. No conclusions whatsoever about the radial extent of the spreading ring sector, that is to say in the ejection direction, can be drawn from the number of impacts either. The centering angle of the spreading ring sector cannot be detected with only two positionally fixed sonotrodes either. Consequently, overall considerable inaccuracies occur when detecting the actual state by sensor.
In a similar but purely visually operating method (DE 14 57 863 A1), the sensors are embodied as pivotably suspended impact plates which are deflected when the particles of granular material impact, with the result that the user can determine visually whether the plate has been impacted by particles. In this way, the user can approximately gauge the radial boundary of the spreading ring sector and appropriately set the disc spreader when undesired deviations occur. Otherwise, what has been stated above with respect to EP 0 682 857 A1 very largely applies.
Furthermore, it is known (EP 0 303 325 A1) to distribute a plurality of sensors on the circumference of the distributor disc in order to display to the driver the position of the ejection range on the distributor disc and subsequently correspondingly intervene in the control in a manual or automatic fashion. The document leaves open how these sensors operate and which measurement variables are recorded in the process. In all the abovementioned cases, only the position of the ejection range, and in the case mentioned first the position of the maximum quantity, is determined and corrective intervention is carried out in the case of deviations. On the other hand, neither the actual position and extent of the spreading ring sector, i.e. that range in which the particles are actually deposited on the ground, nor the mass distribution which is present there is detected. Instead, it is assumed that the ratios which are present or detected directly at the circumference of the disc are equally present on the ground, which does not correspond to the facts.
In a two-disc spreader of the design mentioned last, which is also known, not only the ejection range is detected by means of sensors, but also the quantities of granular material which is actually spread are detected (EP 0 287 165 A1) in that the container with the granular material is weighed using a subtraction method, that is to say the granular material which is discharged per time unit is detected. As a result, although definitive information is obtained about the quantity of granular material discharged, such definitive information is not obtained about the quantity distribution in the spreading ring sector. Moreover, the detection of the weight is structurally very complex and also inaccurate on uneven ground because of the dynamic operation of the disc spreader.
In addition, in the case of a single-disc spreader it is known (EP 0 300 580 A1) to make the spreading ring sector wider or narrower by moving the point at which the granular material is placed on the distributor disc. In order to detect the current extent of the ejection range, optical light sensors or acoustic sensors, which operate according to the reflection principle, are arranged on the disc circumference. These sensors detect not only whether particles are actually located in the beam path, but also the density thereof. In the case of deviations of input set point values, the ejection range is correspondingly changed by moving the placing point and/or the quantity of granular material by controlling the metering opening. However, in fact only the ejection range at the disc and not the spreading ring sector is also determined here and the sensory detection of the granular material is subject to considerable inaccuracies.
DE 195 00 824 A1 discloses a further method in which the stream of fertilizer is recorded photooptically in terms of its direction and speed and the quantity of fertilizer is detected and the data which is determined is input into a computer for determining the transverse distribution and distribution accuracy. The fertilizer spreader is to be set according to predefined boundary limits on the basis of the calculated data. In this method, on the one hand measurement is carried out at a location in the ejection range which is not detected, on the other hand, the relationships between the measured values and the settings which are performed subsequently cannot be understood on the basis of the document.
DE 197 23 359 A1 describes a disc spreader of the generic type in the form of a two-disc spreader with a granular material container with metering openings which can be adjusted by means of metering devices, distributor discs which are arranged underneath the metering openings, run around an essentially vertical axis and have thrower blades which distribute the granular material in a spreading ring sector on the ground, as well as having devices for changing the spreading ring sector which are capable of adjusting the point at which the granular material is placed on the distributor discs and/or the thrower wings of the distributor discs or else of changing the inclination of the distributor discs with respect to the normal plane. A method for setting such a two-disc spreader for distributing the granular material in an adjustable quantity over an adjustable working width is described, according to which quantity of granular material (setting of the metering device) and at least one throwing parameter which characterizes the flight behavior of a specific granular material, such as the ejection range, the mass distribution in the ejection range and the average flight distance are stored as set point values in a computer for each working width. By means of a sensor which is arranged near to the circumference of the distributor disc and which is capable of detecting the ejection range in which the particles of granular material leave the distributor disc, the mass distribution of the granular material within the ejection range and the absolute mass of the granular material per time unit are detected as actual values in addition to the ejection range. The current value which characterizes the flight behavior can be determined on the basis of the actual value of the mass per time unit and the setting of the metering device or the size of the metering opening thereof and/or on the basis of the actual value of the ejection range and the mass distribution as well as the position of the device for changing the spreading ring sector. In evaluation electronics, the actual values are compared with the set point values and the metering opening of the metering device is set to the predefined quantity of granular material on the basis of the detected deviations, and the ejection range, the mass distribution within said range and the average flight distance are set to the set point values by means of the device for changing the spreading ring sector.
However, with such a disc spreader which is embodied in such a way it has also been found that the setting parameters of the device for changing the spreading ring sector, generated on the basis of the actual values, detected by sensor, for the respective throwing parameters are subject to considerable inaccuracies which result in correspondingly inaccurate setting of the device for changing the spreading ring sector of the disc spreader, which results in a transverse distribution of the granular material (over the working width) which is not optimum.